What is the ruling on trading in the black market for medicine by selling it at a price higher than its usual price?

The basic principle is that there is no specific limit to profit in Islamic law. Islamic law prohibits monopolies. If people are harmed by rising prices, the ruler is permitted to impose a just pricing. Regulations established for the public interest must be adhered to. As for applying these realities to the current situation, that is left to those with expertise in it.